Sailing vessels have been known and used for thousands of years and seemingly since before the annals of history. Though primarily used as pleasure craft today, sailing vessels are also often used for competitive racing purposes. As such, sailboat designs are being continuously revised to achieve greater speeds and maneuverability. Indeed, sailboat racing requires such a high level of functional sophistication that design and manufacturing concepts are even being adapted from the cutting edge of space technology.
Conventional sailing vessels generally consist of a buoyant hull (or multiple hulls in the case of catamarans or trimarans) upon which one or more masts extends upwardly from the hull to support one or more sails. A downward extension of the hull, called the keel, traverses the bottom of the hull from bow (front) to stern (rear) and functions to stabilize the sailing vessel against the countervalent force of the wind. A ruder used for steering the vessel is positioned below the stern and is controlled with a tiller or wheel from the deck of the hull.
A typical mast consists of a rigid vertical column which is securely attached to the hull and supports an orthogonally-positioned boom extending outwardly from the mast. Early sailing vessels, which comprised the majority of pre-twentieth century naval fleets, are easily recognizable for their majestic appearance. These early vessels often include two or three masts which supported several horizontally-positioned "yards" spaced vertically and which extended across the mast, each yard supporting a square sail. While some modern sailing vessels continue to employ a similar configuration, most others employ a single mast wherein a boom, which functions similarly to a yard, is positioned on the lower portion of the mast. The planar area defined by the vertically-rising mast and the horizontally-extending boom is occupied by a sail, generally triangular in shape. The sail is securely supported by both the mast and the boom.
While the mast remains steadfastly secured to the hull, the boom is permitted to rotate about the mast, allowing the sail to be movably positioned relative to the hull in order to capture the wind from varying wind directions. With sailing vessels in general, translation of the vessel in water depends upon the forces of the wind. The sails are strategically positioned to capture sufficient wind to power the vessel. In an overly simplified explanation of the mechanics of sailing, the captain of a sailing vessel is able to control the direction of movement by simply steering the rudder with the steering wheel or tiller. Simultaneously, the captain is able to power the vessel by positioning the boom with respect to the wind direction in order to capture the forces of the wind. With this combination of control, a sailing vessel has virtually 290.degree. of directional freedom with respect to any present direction of the wind. Of course, if the wind changes directions, the vessel may be able to head directly upon a desired course which otherwise required back and forth tacking as will be described below.
For simplification purposes only, assume the wind is heading due south out of the North (0.degree. reference point). A sailing vessel will be permitted to travel from a direction approximately 35.degree. NE to about 325.degree. NW under the power of this particular wind vector. Ordinarily, a sailing vessel cannot travel directly against the wind, i.e., at a heating of 0.degree. N. or within about a 35.degree. arc on either side of the wind vector. If the desired course of travel requires such a 0.degree. heading, the captain must direct the vessel in a back and forth--somewhat "zig-zag," formation, repeatedly between 35.degree. and 325.degree.. This approach is referred to as "tacking" or "beating" and naturally results in a slower effective travel speed than when the vessel is travelling with the wind directly at its back, i.e., at a 180.degree. heading. This latter approach is referred to as "running" and provides the greatest opportunity to capture the full force of the wind.
With any particular wind direction, there is a relative position of the sail to the vessel which is optimal, thereby capturing a maximum amount of wind forces. The wind direction in relation to the desired direction of motion determines the proper sailing attitude. If the wind is behind the vessel when the vessel is headed in the desired direction of travel (i.e., 180.degree.), a virtually straight path can be traversed. In this instance, the sails are preferably extended perpendicular both to the wind and the hull wherein aerodynamic wind forces acting normally on the sail provide the propulsive force. When it is desired that the vessel be directed in a heading oblique to the direction of the wind, but at an angle greater than 35.degree., the sail or sails are "trimmed," i.e., they are pulled closer to the longitudinal axis of the vessel. If the wind is blowing at an angle less than about 35.degree. from the desired course of travel, the sail or sails must be fully trimmed and brought as close to parallel with the longitudinal vessel axis as possible. Intermediate sailing attitudes are termed close reach, beam reach and broad reach, with a close reach being closest in sailing mechanics to "beating," described above. Under close reach and beating approaches, the sails function not only to capture the wind directly, but also function effectively as air foils. With conventional air foils such as the wing of an airplane, the force of wind passing over and under the wing provide aerodynamic lift. Analogously, the wind passing on either side of a sail provides forward propulsion because the sail is vertically oriented as opposed to a wing which is horizontally oriented. Much like the airplane wing, sail lift occurs when the wind speed over the leeward (downwind) side of the sail is less than the wind speed over the windward (upwind) side of the sail, resulting in a slight pressure differential across the sail. This pressure differential provides the force necessary to propel the vessel forward.
When the vessel is steered directly into the wind during tacking approaches, the sail or sails temporarily go limp, or luff, in mid-turn. As the vessel continues to "come about," the sail is renewably filled with wind which forces the boom to the other side of the vessel. Likewise, when turning away from the wind, called jibing, the sail is filled on one side of the vessel and suddenly forced to swing to the opposite side in one rapid change of wind direction.
As indicated above, typical single-masted sailing vessels feature a singular triangular main sail pivotable around the mast. Since the amount of power resulting from wind forces on a sail is directly related to the sail area capturing the wind, a rectangular or trapezoidal sail with the same base dimension as a triangular sail offers greater area and thus provides a greater propulsive force on the vessel. Numerous embodiments of fixed-mast, triangular sail vessels may be found in the prior art. For example, the concept of a rotatable peripheral frame with triangular sails is depicted in U.S. Pat. No. 3,195,494 to Robin. There, advantages of enhanced sail trimming and the ability to jibe safely are disclosed.
Another triangular sail configuration upon a rotating mast is shown in U.S. Pat. No. 3,968,765 to Menegus. In the Menegus device, there is no peripheral frame but the advantages gained from relative sail-hull rotation are discussed. Normally the boom is not fixed with respect to the hull and a constant desirable angle of attack towards the wind is preserved due to balancing wind moments about the pivot. This automatic following of wind shifts is termed vaning. Another vessel having triangular sails mounted on a rotatable mast is shown in U.S. Pat. No. 3,802,371 to Jastrab. In this design two identical sails are disposed symmetrically on either side of the main mast and connected together by spars at their top and bottom. At any sailing attitude other than running, one functions as a foresail and the other as a mainsail.
Sailing vessel artisans, recognizing the advantage of greater sail area, have incorporated rectangular sails on sailing vessels. In U.S. Pat. No. 685,943 to Pool, the concept of two rectangular sails disposed on either side of a main mast is described. The sails are held in separate frames which have limited rotation about the mast; however, they also have some longitudinal freedom of travel and may pivot about their extended vertical edge. A similar configuration, but with only one rectangular sail within a rotatable mast, is disclosed in U.S. Pat. No. 4,911,093 to Estrup. Tension cable supports distribute the loading about the mast so that a lighter, less sturdy structure is necessary. Additionally, the mast and horizontal booms are shaped to channel the wind more effectively onto the sail plane, reducing dead spots and tip vortices. U.S. Pat. No. 4,506,620 to Gerr discloses a more sophisticated rectangular sail/rotating mast assembly. At least one rotatable mast holding rectangular sails allows the boat to exploit fair winds (from behind) by extending the sails perpendicular to the wind. When sailing windward (into the wind) the sails are aligned more along the keel axis and thus converted to a fore-and-aft rigging.
Another area of sailboat design includes substituting airfoils for the typically pliant sailcloth. As described above, in almost all sailing approaches except "running," a conventional sail functions at least in part like an airfoil. An airfoil has a shape described by its camber (curvature) and its thickness. A neutral-camber foil is shaped like a thin symmetric teardrop, as shown in U.S. Pat. No. 4,685,410 to Fuller. There a wing-sail which consists of two vertically disposed neutral foils which are rotated about one central mast in the manner of a conventional sail is described. This concept is also disclosed in U.S. Pat. No. 3,332,383 to Wright, except that the latter also discloses variable camber foils. In the Wright patent, the foils are structurally supported by vertical masts which are pivotable thereby changing the cambers of said foils. The masts are then rotatable about a platform which is rotatable about the base of the vessel using either motor or manual drive mechanisms. An example of a plurality of air foils being rotatably supported from a lateral base frame is disclosed in U.S. Pat. No. 4,116,151 to Guthrie. The frame pivots about a vertical axis, as do the air foils within the frame, which provides for maximum freedom to orient the air foils with respect to the wind.
Despite the continuing developments of sailing vessels, there is still lacking a system for employing a plurality of rectangular or quadrilateral sails which are easily operated and functionally practical. While rectangular sails have been employed in the past, there has been missing in the prior art the ability to maneuver a plurality of rectangular sails in a manner analogous to conventional triangular sail vessels. The present invention addresses this void in the prior art.